25th April: ANZAC Day, a public holiday in Australia and New Zealand. Dawn and morning commemorative services at war memorials and other places in both countries as well as in Gallipoli in Turkey.
Buddha's birthday: The Buddha's birth, enlightenment and passing beyond life and death are all celebrated in Cambodia, Laos, Myanmar, Sri Lanka and Thailand, on the 15th day of the 6th month of the lunar calendar - which is usually in April according to the Gregorian calendar.
29th April: Greenery Day (みどりの日 Midori no hi), Japan
30th April (or Saturday 29th if 30th falls on a Sunday): Queens Day, Netherlands
27th April: Freedom Day in South Africa. In celebration of South Africa's first non-racial democratic elections in 1994.
6th April: Chakri Memorial Day is a public holiday in Thailand
13-15th April: Songkran สงกรานต์ is the Thai New Year (celebration dates are fixed, but vary slightly regionally)